The present invention generally relates to a terminal apparatus for use in a digital subscriber line transmission system based on full-duplex transmission and, more particularly, to an adaptive type echo canceller in such a terminal apparatus. In particular, the present invention relates to a terminal apparatus which comprises an adaptive type echo canceller made up of a finite-duration impulse-response (FIR) filter and an infinite-duration impulse-response (IIR) filter.
In a digital subscriber line transmission system for full duplex transmission of digital data between terminal apparatuses through 2-wire transmission lines, each of the terminal apparatuses includes a transmitting line, a receiving line and a hybrid circuit for connecting the transmitting and receiving lines to an external full-duplex transmission line. When a transmission signal to be transmitted is applied to the hybrid circuit within the local terminal, this causes impedance mismatching of the hybrid circuit, thus generating an echo that is undesirably bypassed to the receiving line. To avoid this, there has been suggested such a technique that an echo replica is generated based on the transmission signal of the local terminal to cancel the echo appearing on the receive line as disclosed in U.S. Pat. No. 4,131,767 (issued on Dec. 26, 1978). In this prior art, an echo canceller comprises a finite-duration impulse-response (FIR) filter, and tap coefficients of a plurality of taps of the filter are converged on the basis of an error signal which is generated based on a receiving line signal received from a remote party terminal.
Also disclosed in U.S. Pat. No. 4,868,874 (issued on Sep. 19, 1989) is an arrangement which comprises a combination of an FIR type filter for suppressing an echo impulse response of a short time constant and an IIR type filter for suppressing an echo impulse response of a long time constant, i.e., an echo tail. Another arrangement is disclosed in an article in IEEE Globe'Com '88, 25.2, pp. 778-782, entitled "Design Techniques And Performance of An LSI-Based 2B1Q Transceiver" (1988) that an FIR type filter comprises a plurality of stages of delay circuits connected in cascade and a tap output obtained from the last stage of delay circuit is used as an input of an IIR type filter.